also, on crabs. I have 3 emerald crabs in the tank, smallest one is about 1.5" across (spread out, so it's pretty small), and the big one is about 3" across (again, spread out, so it's a decent size).
the brittle star shares the same cave with the small crab, and has never made any signs of aggression toward it. i've even see the emerald pick stuff off the star's retrieving arm.